Luminescence evolution from alumina ceramic surface before flashover under direct and alternating current voltage in vacuum

Self Cite

Surface discharge in a vacuum occurs under an intense external field, also called a flashover. The flashover threshold over which the surface flashover occurs is of vital importance in the vacuum-solid interface. It is widely assumed that the flashover is initiated by a single surface multipactor and develops as a plasma discharge along a dielectric surface, namely the secondary electron emission avalanche (SEEA). Yet the SEEA model is not versatile for all practical vacuum-solid interfaces and there remain experiment results contradicting the theory predictions without a resolute answer. Here we further expand on previous theories and consider the conventional SEEA model under arbitrary field distributions. The example of a conical insulator is then given to test the validity of the SEEA model under an arbitrary field. It is found that the obtained flashover threshold is only consistent with the experiment at low cone angles where the field near the cathode triple junction is strong, while remarkable discrepancies appear when the anode field is significantly enhanced. The discrepancies can be understood if a different discharge model, i.e. an anode-initiated flashover (AIF) is included. Both particle-in-cell simulation and optical diagnostics are employed to visualize this unique discharge, which are then formalized on theoretical grounds to elucidate the early stage of the AIF. The transition between two flashover phases is then clarified. The plasma-surface interaction and sheath dynamics during the discharge are analyzed theoretically, showing distinct sheath evolutions in two flashover models. Additionally, a new factor reflecting field distortion is introduced to estimate the discharge threshold under an SEEA framework.

Section: Anode-initiated Flashover (Aif)mentioning

confidence: 68%

Estimation of surface flashover threshold in a vacuum II: flashover phase transition

Sun¹,

Guo²,

Li³

et al. 2019

Self Cite

“…In our previous studies, we validated that the SEE process on sample surfaces resulting from field emission at the CTJ leads to surface luminescence phenomena and surface charge accumulation synchronously [46]. Therefore, the surface luminescence of the HR insulator was further investigated by the ICCD camera, as indicated in figure 14.…”

Section: Surface Charge Accumulation and Flashover Characteristics Wi...mentioning

confidence: 68%

Mechanism of vacuum flashover on surface roughness

Guo¹,

Sun²,

Zhang³

et al. 2019

Self Cite

An investigation is conducted regarding the influence of surface roughness on the flashover strength of an insulator in vacuum. A series of experiments is first presented, showing the relationship between surface roughness and flashover voltage, combined with measurement of surface potential distribution. It is found that surface charging on a roughened dielectric is suppressed remarkably; also the flashover voltage threshold depicts a rise-and-fall trend with roughness increasing, exhibiting a voltage summit at a certain roughness value. In addition, optical observation is implemented to draw a parallel with an electron trajectory in vacuum. A proposal can therefore be made that multipactor expansion tends to be mitigated as electron bombardment on a dielectric occurs with a lower secondary electron yield. Then a theoretical model considering the microscopic physical process is established to explain the above phenomena, consisting of an internal charge migration layer, an interfacial electron absorption layer and an external electron obstruction layer. The validity of this theoretical model can be confirmed by obtaining the surface trap distribution, microscopic morphology and net secondary electron yield.

“…To begin with, the SE avalanche starts with the field emission at the CTJ due to field emission, which produces seed electrons. At typical applied electric field levels of vacuum flashover, the Fowler-Nordheim effect is a dominant contributor to the cathode emission current, supported by optical diagnostics of Su et al [36]. The cathode current in the simulation is calculated as follows [37]:…”

Section: Simulation Model Setupmentioning

confidence: 93%

Metal-wire-embedded alumina insulating material using micro- and nanoscale 3D printing for surface flashover mitigation in a vacuum

Mu¹,

Yao²,

Zhang³

et al. 2022

Micro and nanoscale 3D printing technic is applied to fabricate functional insulating material which mitigates surface discharge in vacuum based on the microscopic electron multipactor suppression. The proposed alumina ceramic insulator design consists surface-embedded thin metal wires which introduce a local gradient of secondary electron emission yield, such that the trajectories of multipactor electrons are distorted by accumulated negative surface charges and the secondary electron emission avalanche across the insulator surface is intermitted. Considerable increases of surface flashover threshold and surface charging reduction are verified by experiment. Also, additional efforts are made to determine the optimal size and spatial distribution of the metal wire. A convex-shape flashover voltage trace is observed when increasing the wire width, suggesting a trade-off between the multipactor mitigation and the insulator strength. Wire position between the adjacency of cathode triple junction and middle of the insulator is proved to be favorable for flashover mitigation. The physical details of surface flashover mitigation by the proposed insulator design are revealed by an ab initio particle-in-cell (PIC) simulation code, corroborating the experiment from microscopic aspect.